I'm not a scientist, either, but there is more that affects a hurricane's size and strength than just water temperature.  I'm sure there are several other variables that have changed between these two storms.

That said, water temps are still really high all across the gulf, which is why both these storms are able to gain so much strength (compared to, say, Ophelia)

Here's an interesting map from the NHC - it shows water surface temperature anomalies - notice that the Upper East quadrant of GOMEX is cooler than normal, obviously caused by the eastern wind wall of Katrina - the rest of GOMEX seems to be at expected or slightly warmer temps

(presumably the warm water up near Nova Scotia is the result of Ophelia's rain, as the map is 4 days old)

Here's a map from the Navy from yesterday, just for comparison.
you know that the deathwish christians are gonna be expecting to be sucked out of their socks pretty quick if they spot that little cross-shaped patch of hot water, don't you?

it's south at about 40W lat.